In an electronic switching system, for instance, two main boards for call processing are dualized to construct a duplex board system for system reliability. The duplex board system includes two boards which are functionally identical each other, one of the boards, which is in an active state, performing the function thereof while the other is in a standby state. The stand-by-state board takes over the function of the active state board when the active board fails to function properly due to a defect developed therein.
Referring to FIG. 1, there is provided a conventional duplex board system 10 which comprises two identical processing boards, a side_A board 1 and a side_B board 2.
A state signal ACT*_A through a line L1 depicts the state of the side_A board 1, wherein * denotes that a logic low state depicts the active state. And the state signal ACT*_B through the line L2 depicts the state of the side_B board 2.
It is assumed that the side_B board 2 currently performs the call processing in the active state, i.e., the ACT*_B presents a logic low, and the side_A board 1 has been installed now. When power is applied to the side_A board 1 from a power supply (not shown), the state of the side_A board 1 becomes at first stand-by, i.e., the ACT*_A shows a logic high.
If a defect is developed within the side_B board 2, the ACT*_B becomes logic high to represent the stand-by state. When the side_A board 1 receives the state signal ACT*_B of the stand-by, the side_A board 2 shifts to the active state, i.e., the ACT*_A becomes logic low. Therefore, the side_B board 2 can be removed from the whole system to repair the fault therein, without interrupting the call processing since the counterpart side_A board 1 takes over the call processing. The side_B board 2 is installed after being repaired and waits in the stand-by state as did the side_A board 1 before.
However, when one of the boards, e.g., the side_A board 1, is removed or installed, the state signal ACT*_A may have a glitch period which makes an undesirable state transition at the counterpart board. Accordingly, it is desirable to minimize or prevent the glitch period of the state signal between the boards of the duplex board system.